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Chapter 3 / Sensors / Fins
_ _
and 24° respectively. The constant bias (FNδB) is initialised from a Gaussian
distribution with a deviation of 0.2°. The Gaussian noise has a PSD of
25x10 -6 deg 2 /Hz at the reference frequency, equivalent to 0.1° at 400 Hz.
The constant, quadratic and asymmetric scale factor deviations are set to
0.003, 0.001 and 0.001 respectively. An additional hard travel limit equal to
the output interface range is controlled by bit 11 of MS_FN_ER.
~ AD ⎛ AD ⎛ AD
⎞
⎜
⎛
⎞
⎜ ⎜ ⎜
⎛ A ~
⎟
⎞
AD
δ : = ϕ ϕ ϕ ϕ δ ⎟ , FNf
O ⎟ + ϕ
LIM
Q
ZOH
QN
⎝ ⎝ ⎝ AA ⎝ ⎠ ⎠ ⎠
3.10-4
⎞
⎟
⎠
Equation 3.10-10
The analogue measurement is converted into digital form using a 100 Hz, 4 th
order Butterworth filter, and a 12 bit ADC subject to 3 bits of noise with a
sampling rate and range matched to the output interface.
3.10.5 Sensor Limitiations
Provision of fin deflection measurements at the output interface depends on:
• User measurement selection ( MS_MSAV2 bits 7-10 cleared )
• Physical travel limiting ( MS_FN_ER bit 15 is set )
• Angular rate limiting ( MS_FN_ER bit 16 is set )
When active, if any of these limitations are violated, bits 7-10 in the
measurement ready flag MS_MSRY2 are cleared. Failure to provide a
measurement at the user’s request is indicated by setting bits 0-3 of
MS_FN_VF, one per fin. If the rate limits of 24° and 600°/s are exceeded
bits 4-7 and bits 8-11 of MS_FN_VF are set respectively.
3.10.6 Output Interface
The measured fin positions pass through a signed 16 bit digital interface at a
frequency (FNfO) of 400 Hz (clock 9). The measurements are subject to an
interface range limit of 24° and a one-cycle transmission delay.
FN
~
δ
: =
IF
ϕ
TD
IF IF IF ~
( ϕ ( ϕ ( ϕ ( δ ) ) ) )
Q
LIM
ZOH
Equation 3.10-11